For convenience of expression, following specification and claims use the terms "row, column, horizontal, vertical, and matrix" to describe an orthogonal array. These terms do not necessarily identify any particular orientation other than the orthogonal relationship. Also, the invention is described below in terms of MOS field effect transistors. However, it should be understood that other types of transistors may also be used. Therefore, in the following specification and claims, the references to MOS field effect transistors should be construed to mean any suitable transistor.
A fingertip is an example of a device for applying a very small-surface pressure distribution in a fixed pattern. In a conventional fingerprint pattern detecting apparatus, a fingertip is brought into contact with one surface of a prism which is irradiated with light. The light is reflected with reverberation from the prism surface and is received by a photo-detector element, such as a CCD (charged couple device). The fingerprint pattern is detected in accordance with an output signal from the photo-detector element.
However, this method is susceptible to adverse influences, such as those resulting from sweat and moisture, to an extent that a fingerprint pattern cannot be accurately detected and read. The sweat of the person who had his fingerprint last measured may still be on the surface of the prism to cause measurement errors, resulting in false readings and inconvenience to the person whose fingerprints are being read.
In greater detail, the residue may absorb light irradiated from outside for measurement of a finger print. Because of the residue, there is little reflection of the light from the surface of the prism, resulting in failure of measurement of the finger print. There may also be many other causes of false readings. A failure of the measurement of a finger print may not only be due to the residue from the sweat of a person who previously measured his finger print, but also may be a result of moisture from anywhere, such as ambient humidity, rain, or the like. Dryness may also cause false readings due to a failure of the measurement to correctly detect a total reflection of the light from the surface of the prism. Therefore, a finger print pattern cannot always be accurately detected by a light reading method.
In addition, this prior art method requires a high power consumption and, therefore, is not suitable for battery powered readers. Thus, the described prior art fingerprint readers are not suitable for outdoor measurements, as by police who are working in the field.
In Japanese Patent Laid-Open No. 63-204374, the present inventor disclosed a detecting apparatus for reading a fingerprint responsive to the differentials of pressure or contact resulting in a surface pressure distribution detecting apparatus which was free from the above-mentioned problems of light reflection. According to this disclosure, the pattern is found from a change in conductivity caused by a pressure difference applied by the tops ("ridges") and bottoms ("valleys") of a skin surface forming a fingerprint. The detector responds to the pressure differences on a sheet of conductive rubber having a conductivity which is changed in accordance with the pressure. A scanning electrode matrix responded to the conductivity changes by giving signals in an on/off or digital manner.
The present inventor next described a surface pressure distribution detecting element in Japanese Patent Application No. 2-179735 and U.S. Pat. No. 5,079,947. In this element, scanning electrodes extend in one direction on a hard substrate. The electrodes are spaced apart from each other at a predetermined pitch (50 to 100 .mu.m). Resistance films are formed over the scanning electrodes. Stacked on the resistance films is a flexible film having a lower surface with scanning electrodes extends in perpendicular directions. The total resistance of the resistance films between the scanning electrodes is changed in accordance with the areas of the ridges of the fingerprint skin surface brought into contact with the resistance films, via the flexible film.
The inventor's prior surface pressure distribution detecting elements are free from the adverse influences of sweat and moisture. However, these elements face problems relating to material, structure, and manufacturing. It is difficult to find a practical and durable flexible film material which can accurately transmit the pressure distribution of the skin surface and can assure a deposition of the scanning electrodes, as by etching or the like. During the manufacturing process, it is also very difficult to position the scanning electrodes in rows and columns extending perpendicular to each other. Also, the pressure sensitivity of the element may vary depending on the lack of uniformity of quality.